810 



MORPHOGENESIS 



In Z, alternans, however, hypertrophy of the inferior branch cells begins 

 before the conjugant undergoes its first division. It is therefore doubt- 

 ful whether any increase in the energy requirements, coincidental with 

 the conversion of an apical cell into an exconjugant, is adequate to 

 account for the far-reaching alterations of the normal growth pattern. 

 It is more than likely that the combined energy demands of the ac- 

 tively dividing cells on subordinate branches exceed those of the single 

 conjugant or its first few non-feeding descendants. It is probable, then, 

 that the flux would be directed away from the cell or cells in the 

 apical position. 



Figure 187. A, branch C of a colony fifty-six hours after injury to the neuromuscular 

 cord (drawn from above) (the original colony of six branches was pinched in the mid- 

 region, isolating ABC from DEF and the apical cell ; the terminal cell on branch C dif- 

 ferentiated into a new apical cell, which produced two new branches as illustrated) ; B, 

 schematic representation of branch C, as drawn in A. 



We are thus confronted with two divergent interpretations relative 

 to the specificity of form-regulating factors in Zoothamnium. The evi- 

 dence presented by Summers suggests that qualitative physiological 

 changes in one cell play a greater part in the development of neighbor- 

 ing cells than heretofore suspected. Furssenko's hypothesis, on the other 

 hand, emphasizes the directive influence of metabolic fluctuations. It is 

 therefore consistent with the metabolic-gradient theory, inasmuch as the 

 nutritive factors may be quantitative and continuous. 



The stalk structure of Z. alternans, when viewed in section, somewhat 

 resembles a sheathed nerve fiber. There is an elastic surface membrane, 

 a thick cortical region of hyaline, gelatinous material, and a core of 

 protoplasm, the neuromuscular cord. The latter is continuous from 

 branch to branch and from cell to cell. Unlike the axis cylinder of a 



